(a) Misregistration scan-axis variation—As shown in the Boleda patent documents listed above, image-registration problems can arise from an imperfection in carriage guide mechanisms that cause registration to vary reproducibly along the printhead scan axis. Detecting and measuring these imperfections is the focus of the present document.
The Boleda documents taught that tiny nonlinearities in guide bars, followers, and other components caused minute printhead rotations—leading to errors in registration. Boleda also showed that these errors could be detected by printing and analyzing a test pattern, and compensated by selectively tuning the relative timing of mark generation along different segments, respectively, of the scan path.
Boleda's analysis employed an already-onboard line sensor, provided on the printhead carriage for use in interhead alignment. He commandeered that sensor into further service to detect expansions and compressions of the test pattern, varying along the printer scan axis—due to the above-mentioned mechanism imperfections and resulting fine rotations.
Misregistrations of the sort under consideration—i.e., due to PPS phenomena—can appear as between colors, and also for the same color as between marks made while scanning in opposite directions, and furthermore even for the same direction and color as among marks made while scanning at different speeds. Errors also can arise as combinations of these effects.
Mechanical imperfections leading to such misregistrations can in turn arise as imperfect straightness in a guide rod itself, or imperfect planarity or cylindricity of a platen or the like which establishes the nominal printing-medium position, or imperfect parallelism between the rod and the print-medium position. Typically a guide mechanism itself has plural members, and imperfect geometries between or among those can produce a twisted form of error that is sometimes of one sense and at other times of opposite sense. Imperfections also can arise as combinations of all these effects.
The Niikura document, too, mentions mechanism problems leading to registration variations, but those variations run perpendicular to the scan axis. In his brief discussion of scan-axis variations, Niikura is concerned only with another misregistration source (printing-medium curl) that is not of interest here. Niikura thus suggested no connection between his compensation for scan-axis variations and any built-in hardware errors.
In addition, to the extent that Niikura investigated any registration variations along the scanning axis, his principal method of assessing such variations relied upon very expensive acquisition of electronic images of preprinted hardcopy regions—using a charge-coupled detector (“CCD”), and then computation-intensive processing to compare halftone dot sizes or spacings.
A typical CCD, as is well known, is an expensive multipixel device that yields an actual image of the preprinted hardcopy region; and Niikura's acquired image is very greatly enlarged to permit extremely fine analysis of minute image details. (Other Niikura teachings involving a “laser sensor”, for variation transverse to the scan axis, are ambiguous as to both the character of the sensor and methodology of its use; possibly it was interferometric.)
(b) Measurement methods and their drawbacks—Boleda—and also Niikura, in dealing with registration fluctuations along the scan axis—depended upon analysis of some information premarked on the printing medium. Boleda used a simple and essentially free device already present in the printer; Niikura used the above-mentioned CCD—a relatively very costly device—and also elaborate, sophisticated postprocessing.
Each of these earlier systems has its respective definite limitations. The Boleda approach requires preprinting on the print medium something that would not otherwise be printed—and this consumes medium, ink, and time. Niikura's approach for scanwise error (due to cockle) minimizes this drawback by scanning a previously printed portion of an in-progress hardcopy; but his analysis stage requires expensive componentry and heavy computation.
What is desired is some way to measure departures from uniform printhead-to-print-medium spacing without printing, without special equipment, and without significant signal processing. Heretofore no such way has appeared in the art.
(c) Factory PPS determination—The Boleda patent document first-mentioned above shows how rectilinearity of a carriage guide bar can be evaluated through printing and analysis of a test pattern. Entirely apart from the cost, delay and inconvenience of generating the test pattern to obtain these relative measurements all along the scan axis, another severe prior-art limitation is the difficulty of obtaining an absolute value of PPS at even any single point in the path.
Such an absolute measurement, at least at some single point, is an additional piece of data requisite to trustworthy PPS calibration. Heretofore such a measurement has been possible only through positioning some special measuring fixture in the printer, or a special jig next to the printer, to perform an actual primary determination.
After this determination has been completed, furthermore, the jig or fixture must then be removed carefully to ensure its continuing good condition for further accurate measurements of other printers. These factory equipments and operations add up to a significant and undesirable manufacturing cost and complication.
(d) Machine printing formats: scanning-head and pagewide-array, and equivalents—The documents mentioned above deal with printers in which relatively small marking heads (“printheads”), whose length is only a fraction of the height of the desired image, are mounted on scanning carriages that traverse the width of a desired image area. Marking is accomplished by operating the heads during such scanning, to form a swath of marks; then the printing medium is advanced in the orthogonal direction, to position the medium relative to the head for forming the subsequent swath.
Another type of system that suffers misregistration arising from PPS variation is a so-called “pagewide array” printer. In this type of machine, an array of marking elements (for each color respectively) extends across the entire image-area width; this array prints an entire line while the printing medium is advanced in the orthogonal direction—thereby forming an entire image in (most typically) a single pass of the medium through the printer.
The term “pagewide array” arises from the initial use of such systems to print on small-format sheets such as, for instance, A4 pages or 8½×11-inch pages. Equivalent operation is of interest in large-format printers, but these perhaps may not be properly denominated “pagewide”—since many of these large-format machines are loaded with rolls of paper rather than page-size sheets.
Naturally in such pagewide-equivalent units a sheet is eventually formed when a length of the roll is cut off after printing. The PPS-variation problem is a major concern in pagewide-array machines—and their large-format equivalents just discussed—as well as in scanning-head printers.
(e) Conclusion—Relatively cumbersome, expensive or slow strategies for measuring scanwise-varying misregistration due to mechanism imperfection have continued to impede achievement of uniformly excellent and rapid ink-jet printing. Thus important aspects of the technology used in the field of the invention remain amenable to useful refinement.